Different such display devices are known. By touch control is here primarily intended a control where the operator touches the device with his finger.
Accordingly, there are known such touch controlled display devices where control is exercised by the pressure action to which the device is subjected when the finger touches the device. Examples of pressure controlled display devices are mentioned in the Xerox Disclosure Journal, Vol. 5, No. 5, September/October, 1980, page 559, and in the German Offenlegungsschrift 30 39 713, as well as in the Japanese published specification 56-161521. In the latter there is utilized the capacity change occurring with pressure on the display device, by the distance being altered between detection electrodes and a counter-electrode, these electrodes being disposed on the inside of the respective front and back cover plates of the display device.
These known pressure controlled display devices have troublesome disadvantages, since the cover plates must be elastic in order to be activated by the touch pressure and to return to their initial state after being touched. This elasticity is troublesome, particularly with display devices having a large surface, in which a touch easily results in that the electrodes come into direct contact with each other. Furthermore, repeated pressure loading results in risk of the cover plates becoming fatigued. Operational reliability in such known pressure controlled display devices has been found to be small.
Another type of touch control is carried out with the aid of direct capacitative detection of the finger touch on the display device. In this case, separate touch contacts are placed on the outside of the cover plate or window of the display device. This type may be exemplified by the devices described in the British Patent Applications 2 066 548 and 2 094 050 and the German Offenlegungsschrift 29 32 540.
All the touch control display devices known up to now have required separate detection or touch contacts, thus making the circuitry complicated due to the extra contacts requiring extra wiring. An advantage with disposing the detection contacts on the inside of the transparent window, eg in accordance with the cited Japanese publication, is that the problems of wear and dirtying which trouble all other known devices are dispensed with. However, apart from the mentioned disadvantages there is also a very serious problem with the known devices with separate detection contacts on the inside and outside of the window, namely that these separate contacts, in themselves and due to extra wiring, are extremely voluminous. The separate contacts must namely be disposed at a given distance from each other and from the customary displaying means in the display device. Wiring will be complicated, since the wires from the separate contacts must be accomodated together with other feed and control wires fro the customary displaying means.